1. Field of the Invention
The present invention relates to a high-yield-ratio high-strength steel sheet (cold-rolled steel sheet, hot-dip galvanizing-coated steel sheet, and hot-dip galvannealing-coated steel sheet), and in particular, to a high-strength steel sheet having tensile strength of 980 MPa or more, in which a yield ratio is enhanced without a decrease in workability.
A steel sheet according to the present invention can be preferably used in: automotive structural members each of which is required to have high-workability and a high-yield ratio (e.g., body skeleton members such as a side sill, pillar, member, and reinforce; and reinforcement members such as a bumper, door guard bar, seat part, and suspension part); home appliance members; and the like.
2. Description of the Related Art
With the growing consciousness of global environmental problems, each automotive manufacturer has recently been promoting weight saving of car bodies for the purpose of increasing fuel consumption. Further, from the viewpoint of the safety of occupants, the crash safety standards of vehicles are tightened and the durability of a member against an impact is also required. Accordingly, the use ratio of high-strength steel sheets has been further increased in recent vehicles. For example, in car body skeleton members and reinforce members each of which is required to have a rust-prevention property, hot-dip galvanizing-coated steel sheets and hot-dip galvannealing-coated steel sheets (hereinafter, sometimes represented by coated steel sheets) having high-strength are positively adopted.
The aforementioned steel sheets are required to have excellent spot weldability, excellent workability, and an energy absorption ability upon crash, and also required to have high-yield strength, i.e., a high-yield ratio.
From the viewpoint of increasing spot weldability, a reduction in the amount of C is effective, and, for example, in Japanese Unexamined Patent Publication No. 2007-231369, a steel sheet in which the amount of C is remarkably reduced to less than 0.1% is used. However, if the amount of C is reduced, the steel sheet has low-yield strength while having excellent workability, such as ductility, thereby causing the problem that high-yield strength and workability cannot be achieved together.
In addition, in Japanese Unexamined Patent Publication No. 2002-322539, a thin steel sheet is disclosed which contains C of less than 0.10%, is substantially made of both a matrix having a ferrite single phase microstructure and fine precipitates each having a grain size of less than 10 nm dispersed in the matrix, has tensile strength of 550 MPa or more, and has excellent press formability. According to the example in Japanese Unexamined Patent Publication No. 2002-322539, however, the tensile strength of the thin steel sheet is at most approximately 810 to 856 MPa, and it is not disclosed that even a high-strength steel sheet having tensile strength of 980 MPa or more can be made to have both high-yield strength and excellent workability.
On the other hand, an example of a steel sheet having both high-strength and workability includes a dual-phase steel sheet (DP steel sheet) whose major components are ferrite having high-elongation and martensite exerting high-strength. In the DP steel sheet, however, only a low yield ratio can be obtained, and hence a high-yield ratio and high-workability cannot be achieved together. As the aforementioned DP steel sheet, high-strength hot-dip galvanizing-coated steel sheets each having excellent strength-ductility balance are disclosed in, for example, Japanese Unexamined Patent Publications Nos. Sho 55 (1980)-122820 and 2001-220641. In these methods, however, martensite is generated in a cooling step after a hot-dip galvanizing-coating treatment or an alloying treatment, and mobile dislocation is introduced in the ferrite during the martensite transformation, and hence yield strength becomes low.